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Research published today in the journal Nature suggests that fewer small asteroids
will collide with the Earth to produce craters than previously thought. The research, by
Dr Phil Bland, of the Department of Earth Science and Engineering, Imperial College London and
Natalya Artemieva of the Russian Academy of Science, has modelled how asteroids break apart in
the Earth's atmosphere due to the tremendous forces they experience.
Previous calculation of asteroid fragmentation in the atmosphere have relied on a "pancake" model in
which the asteroid breaks up into small pieces and spreads out to form a pancake-shaped cloud of fragments.
Bland and Artemieva's model treats each fragment separately and they suggest provides a much more
accurate description of how the atmosphere protects the Earth's surface.
"Massive impacts of the type thought to have wiped out the dinosaurs
leave an indelible print on the Earth but we have not been able to
accurately document the effect of smaller impacts. Now, we have a handle
on the size of 'rock' we really need to worry about and how well the
Earth's atmosphere protects us," says Bland.
The results indicate that asteroids with a diameter greater than 200
metres (the length of two football pitches) will hit the surface
approximately once every 160,000 years - way down on previous estimates
of impacts every 2,500 years.
The findings also predict that many more asteroids blow up in the
atmosphere than previous estimates, which means the hazard posed by
impact-generated tidal waves or tsunamis is lower than previous
predictions. The researchers suggest that proposals to extend monitoring
of Near Earth Objects (NEO) to include much smaller objects should be
reviewed.
Bland points out that their new predictions correspond a lot better to the number
of small craters found on the surface of the Earth, but warns that not all small asteroids
will harmlessly explode in the atmosphere.
"Our data show that stony asteroids need to be 1,000 times bigger than the iron ones to make a
similar sized crater. But we are not out of the woods yet," added Dr Bland "asteroids that
fragment in the atmosphere still pose a significant threat to human
life."
More info: Nature
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